Cognitive function in humans declines in essentially all domains starting around age 50-60, and neurodegeneration and dementia seem to be inevitable in all but a few who survive to very old age. Mice with a fraction of the human lifespan show similar cognitive deterioration indicating that specific biological processes rather than time alone are responsible for brain aging. While age-related cognitive dysfunction and dementia in humans are clearly distinct entities and affect different brain regions, the aging brain shows the telltale molecular and cellular changes that characterize most neurodegenerative diseases including synaptic loss, dysfunctional autophagy, increased inflammation, and protein aggregation. Remarkably, the aging brain remains plastic and exercise or dietary changes can increase cognitive function in humans and animals, with animal brains showing a reversal of some of the aforementioned biological changes associated with aging. Using heterochronic parabiosis we showed recently that blood-borne factors present in the systemic milieu can inhibit or promote adult neurogenesis in an age-dependent fashion in mice. Accordingly, exposing an old mouse to a young systemic environment or to plasma from young mice increased neurogenesis, synaptic plasticity, and improved contextual fear conditioning and spatial learning and memory. Preliminary proteomic studies show several proteins with stem cell activity increase in old rejuvenated mice supporting the notion that young blood may contain increased levels of beneficial factors with regenerative capacity. In this application we intend to test the hypothesis that blood-borne protein factors in young mice are sufficient to increase adult neurogenesis and regenerate the old brain, and that a focused proteomic screen will allow us to identify the most potent such factors. Our studies pursue the innovative concept that brain aging and cognitive dysfunction is at least in part under control of factors from the circulatory environment and that such factors are sufficient to rejuvenate the aging brain.